Field of the Invention
The present invention relates generally to the field of interferometers and more particularly to a self-referencing interferometer that uses spatial phase shifting to reconstruct optical wave fronts.
Description of the Related Art
Phase shifting interferometers used in directed energy or laser communications applications typically consist of a light source that is split into two beams—a reference beam and a signal beam. The two light beams travel different paths, the reference beam is spatially filtered, and then the beams are recombined in such a way that the wave front of the signal beam can be determined. Phase shifting interferometry can be used to accurately determine the phase differences between the two beams. By spatially filtering the reference beam to create a clean spherical wave front, the shape of the wave front of the signal beam can be determined. The recombined beams produce an optical interference pattern for each of the phase shifts between the reference beam and the signal beam. Prior art interferometers typically use a combination of prisms and wave plates to split optical beams and recombine the beams with different relative phase shifts. The limitations with prior art interferometers are that they can be large in size and can weigh several pounds or more, limiting their implementation. There are a number of technical limitations with prior art interferometers as well.
For example, existing interferometers must be designed to address non-common path aberrations. When a signal beam and a reference beam do not travel the same path within the interferometer, the two beams may be subject to aberrations that are not common to the paths of each beam. Since the interferometer measures the phase differences between the two beams, non-common path aberrations distort the desired measurement. Designing an interferometer to correct for these aberrations adds to the complexity of the interferometer.
As another example, it is not unusual for an interferometer to include numerous wave plates and prisms, each of which typically comprises two optical surfaces. Even with high quality components, as the number of optical surfaces increases, the optical throughput of the interferometer decreases and any non-common path aberrations increase. For example, FIGS. 1A and 1B illustrate a prior art spatial phase shifting assembly for use with an interferometer. FIG. 1A shows a prototype spatial phase shifting assembly 110 and FIG. 1B shows an isometric cross-sectional view of the same spatial phase shifting assembly 120. FIG. 1B shows two separate input light beams 125, comprising a reference beam and a signal beam, entering the spatial phase shifting assembly. The reference beam and signal beam pass through a series of prisms and wave plates that split the beams and combine them into four separate beam pairs, each pair with a different relative phase shift between the reference beam and the signal beam. FIG. 1B shows the four beam pairs 130, each pair comprising a reference beam and a signal beam, exiting the spatial phase shifting assembly 120. A conventional spatial phase shifting assembly, such as the one shown in FIGS. 1A and 1B can comprise 30 or more prisms and wave plates totaling 60 or more optical surfaces. The numerous optical surfaces limit the optical throughput of the spatial phase shifting assembly and worsen the problem of non-common path aberrations. Furthermore, having numerous custom optical elements and a custom mounting assembly in a conventional spatial phase shifting assembly greatly increases the cost of the assembly.
The weight and size of conventional spatial phase shifting assemblies also limits prior art interferometers. For example, the spatial phase shifting assembly shown in FIGS. 1A and 1B typically weights approximately three pounds and is five inches in length. The length of the spatial phase shifting assembly affects the focal lengths of the lenses used in the optical relays of the wave front sensor component in an interferometer. Accordingly, the weight and size of conventional spatial phase shifting assemblies limits how interferometers can be implemented and in which applications they can be used.
Another limitation in conventional interferometers is the length of the single mode fiber through which the reference beam passes. Conventional interferometers typically use a single mode reference fiber to spatially filter the reference beam. In a typical interferometer, the reference fiber is approximately eight inches in length. However, optical path matching is required to maintain coherence between the signal beam and the reference beam and ensure measurable contrast in the interference images. Due to the refractive index of the reference fiber, the length of the reference fiber requires approximately twelve inches of matching optical path and associated optical elements to be added to the signal path. The additional elements increase the weight and size of the interferometer.
Finally, the polarization requirements of a conventional interferometer using a spatial phase shifting assembly such as the one illustrated in FIGS. 1A and 1B further limit implementation of conventional interferometers for directed energy and laser communications applications. For example, the spatial phase shifting assembly in FIGS. 1A and 1B uses polarization to create the four beam pairs which produce the four interference images. The wave front sensor that measures the interference images requires fixed polarizations for the signal beam and reference beam at its input aperture. Furthermore, the length of the reference fiber creates a greater probability that mechanical and thermal disturbances can alter the polarization of the reference beam and reduce the measurement accuracy of the interferometer.